Gabriela is 2 times as old as Christopher. Fifteen years ago, Gabriela was 7 times as old as Christopher. How old is Christopher now?
We can use the given information to write down two equations that describe the ages of Gabriela and Christopher. Let Gabriela's current age be $g$ and Christopher's current age be $c$ The information in the first sentence can be expressed in the following equation: $g = 2c$ Fifteen years ago, Gabriela was $g - 15$ years old, and Christopher was $c - 15$ years old. The information in the second sentence can be expressed in the following equation: $g - 15 = 7(c - 15)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $c$ , it might be easiest to use our first equation for $g$ and substitute it into our second equation. Our first equation is: $g = 2c$ . Substituting this into our second equation, we get: $2c$ $-$ $15 = 7(c - 15)$ which combines the information about $c$ from both of our original equations. Simplifying the right side of this equation, we get: $2 c - 15 = 7 c - 105$ Solving for $c$ , we get: $5 c = 90.$ $c = 18$.